In many lighting systems, an accurate control of the light intensity and light colour would be desirable, for example in office environments or any lighted indoor space in which a uniform lighting effect should be attained, regardless of whether some regions have additional natural light (e.g. window areas) while other regions are only illuminated by artificial light. With prior art lighting systems using white light sources such as incandescent, halogen or fluorescent lamps that generate essentially white light, control is limited to detecting too-bright or too-dark regions in the environment and regulating the corresponding light sources. The distribution of the light in a room or environment is referred to as the ‘angular distribution’ of the light.
For more advanced lighting systems using luminaries that comprise different coloured light sources combined to produce, for example, white light, the control is more complicated since each of the red, blue and green components influence the overall colour-point or colour temperature of the light. Light in a room can therefore have a ‘spectral distribution’ that is not uniform, with different parts of the room being illuminated with light of different colour temperatures.
Some attempts at analysing such spectral and angular light distributions with a view to controlling lighting arrangements have been based on the use of dispersive elements such as prisms or gratings to act as band-pass filters. Here, the spectral distribution is obtained by measuring the light transmission over its angular span. This is generally done by continuously adjusting the geometrical configuration of sensor comprising such a dispersive element, and subsequently projecting the light transmission onto a photodiode or photomultiplier. In order to spectrally characterize a light source using prior art techniques, it is necessary to mount the dispersive sensor in a fixed known position with respect to the light source, and the acceptance angle must be restricted to a narrow cone in order to obtain a well-defined transmission spectrum for the individual filters. However, to obtain an accurate impression of the angular distribution of the light in the room or environment, the light must be collected over a wide angle. Using current methods, a large number of sensors would have to be distributed about a room in order to obtain an accurate determination of the spectral composition and spatial distribution of the light in that room. Evidently, such a solution would be prohibitively expensive as well as complicated, since the outputs of all sensors would have to be compared and analyzed in order to generate appropriate control signals for the light source(s) in the room.
Therefore, it is an object of the invention to provide a straightforward and economical way of accurately determining the spectral composition and spatial distribution of light.